The present invention relates to an imaging apparatus equipped with a solid-state image sensing device having an imaging area divided into at least two areas from which video signals are derived, and a method of optical-black clamping.
With widespread home use of camera-built-in VTRs, gradually spread is a camera-built-in VTR with an electronic still-picture imaging function to process video signals output from a solid-state image sensing device and transfer the processed signals (still-picture data) to several types of equipment, such as personal computers.
The total number of pixels in a solid-state image sensing device has been dramatically increased with development of LSI microfabrication technology.
Most camera-built-in VTRs have an OB (Optical Black)-clamp function.
The OB-clamp function adjusts a black level of a video signal output from a solid-state image sensing device. The clamp function requires rows of several ten black-level detecting pixels in the vertical direction in an imaging area of the image sensing device.
Several pixel signals are extracted from among those black-level detecting pixels. The average level of these pixel signals is set in a signal level of no light output. The set signal level is subtracted from an output signal level from the imaging area to adjust a black level of a video signal.
The number of pixels in each row increases as the number of those on a solid-state image sensing device increases. The clock frequency for signal output from the image sensing device becomes higher in proportion to the increase of pixels. This is because the duration of video-signal output from a solid-state image sensing device per row of pixels depends on video standard such as NTSC and PAL.
Signal output from a solid-state image sensing device at high clock frequency requires the same high frequency for post processing. Such a requirement causes many restrictions on circuit design with noise and radiation suppression.
The clock frequency can be made half by providing a solid-state image sensing device with two signal-output channels for an imaging area divided in left and right.
Such imaging-area division, however, increases the number of signal-output channels for imaging apparatus used in camera-built-in VTRs. Increase in the number of signal-output channels further causes variation in output-buffer characteristics over the channels. This results in stepped signal fluctuation on the border of the output channels. The stepped signal fluctuation will become sharp as the output fluctuates over the output-channel buffers due to temperature rise, which could occur in long-period filming.
Imaging apparatus used in camera-built-in VTRs with right and left imaging areas suffer OB level differences for video signals output from the imaging areas. The main cause of the OB-level difference is variation in imaging characteristics occurring in mass production of solid-state image sensing devices. The OB-level difference causes inaccurate OB-clamp function due to usage of the average of different OB levels. This results in difference in black level between right and left in video.
Moreover, imaging apparatus with multiple pixels require a high frequency such as 36 MHz for drive pulses in charge transfer. Such a high frequency causes unrectangular pulse waveforms and inefficient charge transfer through many transfer stages. This results in OB-level differences over rows of pixels, which should not occur ideally.
Furthermore, a reference black level is set at an average output-signal level from black-level detection pixels over the entire imaging areas in known imaging apparatus. This reference black-level setting could cause inaccurate black-level adjustments due to level difference between the upper and lower zones in imaging areas.
Known imaging apparatus further suffer small modulated light outputs through several color filters even at no light input. Such small outputs do not meet a reference black level set at the average of chrominance signals.